The wake human brain constantly samples perceptual information from the environment and integrates them into existing neuronal networks. Neuronal oscillations have been ascribed a key role in the formation of novel memories. The theta rhythm (3-8 Hz) reflects a central executive mechanism, which integrates novel information, reflected in theta-coupled gamma oscillations (> 30 Hz). Alpha oscillations (8-14 Hz) reflect an attentional gating mechanism, which inhibit task irrelevant neuronal processes. In my dissertation I further scrutinized the oscillatory dynamics of memory formation. Study 1 demonstrated that theta-gamma coupling reflects a specific mechanism for associative memory formation. In study 2, I experimentally entrained memory encoding by visual evoked theta-gamma coupling processes, to underline its functional relevance. In two developmental studies, I found that the theta rhythm indexes explicit learning processes in adults and young children (study 3), and that visually entrained theta oscillations are sensitive to the encoding of novel, unexpected events, already in the first year of life (study 4). Throughout these studies alpha oscillations were not sensitive to memory formation processes, but indicated perceptual (study 1) and semantic (study 3) processes. I propose an integrative framework, suggesting that the alpha rhythm reflects activated semantic representations in the neocortex, while theta-gamma coupling reflects an explicit mnemonic control mechanism, which selects, elaborates and integrates activated representations. Specifically, by squeezing real time events onto a faster, neuronal time scale, theta-gamma coding facilitates neuronal plasticity in medio-temporal networks and advances neuronal processes ahead of real time to emulate and guide future behavior.

It is a common finding that neurons synchronize their firing rates in a high frequency range above 30 Hz, the so‐called gamma band, if they are involved in the processing of the same stimuli. Furthermore, it was hypothesized that object representations are stored within the cortical networks that are formed of synchronously firing neurons. Consequently, induced gamma band responses in the electroencephalogram were utilized to investigate cortical object representations. A more recent finding by Yuval‐Greenberg et al. questioned this common interpretation. It was shown that miniature eye movements as they occur during fixation are accompanied by small artifacts that mimic activity in the gamma band. The question arose whether induced gamma band responses in the electroencephalogram are a marker of neuronal processing or a mere artifact caused by miniature eye movements. The present thesis aims at providing solutions for this dilemma by investigating whether evoked gamma band responses and steady state visual responses are suitable tools to examine cortical network activity in relation to object recognition. Both measurements are not affected by miniature eye movements. It was found that evoked gamma band responses are particularly qualified to study early processes of object recognition. Furthermore, steady state visual evoked potentials were sensitive to the semantic content of presented stimuli. Therefore they are feasible for studying object recognition, too. Additionally, the present thesis provides the COSTRAP algorithm, a new method to effectively identify and remove the eye movement artifacts related to miniature eye movements. The remaining induced gamma band responses are most certainly of cortical origin. They proved to be sensitive to object recognition and repetition priming. Finally, it was shown that miniature eye movements and cortical gamma band responses can be experimentally dissociated, further underpinning the different methodological importance of induced gamma band response when studying object recognition. In summary, it was shown that the investigation of cortical networks representing object knowledge can also be addressed by evoked gamma band responses and steady state visual responses. Furthermore, it was proven that induced gamma band responses remain a powerful tool in electrophysiological research when studying the neuronal dynamics behind object recognition and other cognitive processing.

Approach and Avoidance motivation are two of the oldest psychological concepts of behavior. Whereas approach motivation corresponds to the strong urge to come close to an object, state or person (e.g., during states of sexual attraction), avoidance motivation corresponds to the strong urge to avoid a specific situation (e.g., evade a dangerous situation). This dissertation deals with the electroencephalographic (EEG) markers of approach and avoidance motivation, assessed using event-related-potentials and brain oscillations in a low frequency band (alpha band). The first manuscript shows a left hemispheric processing advantage for approach-related stimuli. Specifically, we report a reduction in the alpha band (as an inverse maker for cortical activity) for erotic, but not for control pictures. Notably, we are the first to report alpha-asymmetries using an event-related design. In the second manuscript we describe evidence (a) for separating approach motivation from the affective dimensions of valence and arousal and (b) for an enhanced attention-related early EEG amplitude (P1 component) only for approach-related but not for control pictures. Up to our knowledge, we are the first to associate the P1 component with approach motivation. In the third manuscript we report an enlarged P1 component for increased avoidance motivation, as measured by the negative affect scale of the German Positive and Negative Affect Schedule (PANAS; Krohne, Egloff, Kohlmann, & Tausch, 1996). In more detail, state negative affect correlated positively with the P1 component, as a marker of increased selective attention. As far as we know, no study ever showed that state negative affect has an influence on attention. Therefore, we consider these findings regarding previous findings on trait negative affect, specifically on anxiety and phobia. All findings are discussed in the context of established views and models, such as Personality Systems Interaction (PSI) theory, hypervigilance theory, wanting versus liking and are also integrated into the findings from neuroimaging studies.

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Classical fear conditioning – including acquisition and extinction – is a model for fear learning and memory in health and disease. Moreover, trauma-related disorders can be viewed as comprising fear acquisition under severe stress. Yet, in humans, we know comparatively little about how acute stress affects fear conditioning. Therefore, the first aim of this thesis was to investigate the effect of stress on fear acquisition or extinction. Stress induces multiple hormonal and neurotransmitter changes dynamically developing over time, including a fast first-wave and a slower second-wave stress response. Models derived from avoidance learning and declarative memory studies suggest that stress effects on memory depend on the temporal proximity between learning and stressor: encoding close to the stressor will be enhanced, but encoding and recall later in time (during the second-wave) will be suppressed (e.g., Schwabe, Joëls, Roozendaal, Wolf, & Oitzl, 2012). So far, these predictions were not related to fear conditioning. Therefore, we investigated if the model-based predictions are also valid in human fear conditioning. We used two stressors to investigate first-wave and second-wave stress effects: the cold pressor test (CPT) inducing a strong first-wave but little second-wave activation and a psychosocial stressor, reliably inducing both, first- and second-wave stress responses. Conditioning was measured via skin conductance responses (SCRs). Investigating the first-wave (Experiment 2), we placed fear acquisition and immediate extinction directly after the CPT (n = 20) or after the control treatment (n = 20). We found no group difference in acquisition performance, but significantly increased extinction resistance in the stressed CPT group. In Experiment 3, CPT (n = 20) or control (n = 20) was placed after acquisition but directly prior to extinction training. Here, we found improved extinction and 24h-delayed extinction recall after CPT. Investigating the second-wave (Experiment 1), we placed fear acquisition and immediate extinction 45 min after the psychosocial stressor (i.e., at the peak of salivary cortisol, n = 12) or after control (n = 12). Here, we found no significant stress effects. Sex and female sex hormones also influence fear conditioning: Women are at a higher risk to develop anxiety and stressor-related disorders than men. Interestingly, patients with these disorders show impaired fear extinction and extinction recall, and low levels of the sex hormone 17β-estradiol (E2) are linked to impaired extinction in both, healthy and patient female samples. So far, there is little data on how acute stress and circulating E2-levels might interact in fear acquisition and especially in fear extinction. Therefore, the second aim of this thesis was to explore this possible interaction in healthy women in different cycle phases compared to men. Thus, in Experiment 4, we included hormone status as a quasi-experimental variable and compared free cycling women in the midcycle phase (high E2, low progesterone, n = 24), women in the early follicular phase of the menstrual cycle (low E2, low progesterone, n = 24), and men (n = 24). We placed fear acquisition and extinction 45 min after the psychosocial stressor (n = 36) or control (n = 36), and tested extinction recall after 24 h. In line with Experiment 1, the second-wave stressor did not affect fear acquisition and immediate extinction. However, we found a stress by hormone status interaction within women at the 24h-delayed extinction recall test: in the stressed group, early follicular women showed impaired extinction recall and a higher return of fear compared to midcycle women, whereas there was no difference between early follicular and midcycle women after control treatment. Collectively our results support a different role for the first- and second-wave stress response in human fear conditioning. Fear acquisition near the first-wave stress response results in enhanced fear memory, which is resistant to extinction. Extinction training near the first-wave enhances extinction learning. In contrast, fear conditioning at the peak of the peripheral second-wave cortisol response had no effect on acquisition or extinction performance. However, second-wave stress interacted with the hormone status of women, where only women in a low E2 state were vulnerable to negative stress effects in extinction recall. The last result will encourage further investigation of the interplay between E2 and stress in fear extinction. Enhancement of extinction by the CPT could – if replicated – be translated into strategies for optimizing exposure therapy.

The aim of this study is to investigate the neural mechanisms underlying the drawing of inferences based on a reader s knowledge during reading. Previous research studies have investigated this topic by using different types of text materials varying in coherence (e.g. Kuperberg et al., 2006), complexity (e.g. Xu et al., 2005), comprehensibility (e.g. Vandenberghe et al., 2002) or acceptability (e.g. Hagoort et al., 2004). Instead of using different types of text materials, we used a less explored method that manipulated the reader s reading goals to vary the level of engagement of inference processes. Cognitive psychologists have shown that the reader s reading goals have considerable influence on the cognitive processes of comprehension and on the content of the resulting representation of the text (Graesser et al., 1994). Here, two functional magnetic resonance imaging (fMRI) experiments were conducted to investigate the neural mechanisms of drawing strategic and routine inferences. The experimental data were analysed using two complementary approaches, namely conventional fMRI data analysis and effective connectivity analysis. Combined with an anatomical model developed in this study, the latter approach enabled us to quantify the interregional interactions modulated by the experimental conditions and to discriminate between several plausible hypotheses regarding how inferences are drawn. The results of both fMRI experiments show that the left inferior frontal gyrus (IFG) in Brodmann area (BA) 45/47 is involved in inference processes, regardless of whether inferences are drawn strategically or routinely, while the left anterior prefrontal cortex in BA 9/10 is only involved in retrieving strategic inferences. The effective connectivity analyses show that the retrieval of strategic and routine inferences consistently enhances the connectivity between the left posterior superior temporal sulcus and the left dorsal lateral IFG.

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